Board Thread:General Discussion/@comment-31398341-20170409101440/@comment-27535813-20170804054555

Mega Gengar has 130 base speed, and Greninja has 122 base speed. Mega Gengar using Focus Blash would outspeed it, and is the only move to check it as you can't afford to lose out coverage against other potential dark-types, unless it has scarf(when choice items are released). Unless it has Battle Bond, Greninja-Ash has 133 base speed which would outspeed Mega Gengar, given that it is even possible for someone to get timid and 31 speed IVs on Greninja-Ash in PBB anyway.

Mega Gengar should be run by having its own threats to it being removed and then trap on a Pokemon you can set up/switch into/attack freely and sweep the team, so if you're worried about Greninja you best get something else to cover it, preferably a Fairy/Fighting-Type Pokemon that can synergise with it, such as Azumarill (checks most Earthquake users that threaten Gengar since Levitate is gone now, and having a fairy-typing which scares dark-types except for pokemon like Bisharp and Alolan Muk), and Breloom (Spore + trapping from Mega Gengar can be pretty useful if your opponent decides to stay in and wait out Sleep Turns, can help punch holes into opposing team w/ nice base attack stat and threatens Dark-Types as well as Steel-Types, and threatens earthquake users; although lack of priority move that hits hard against earthquake users compared to Azumarill with Aqua Jet).

Substitute is my preferred move to replace hypnosis since personally, Mega Gengar won't be affected by statuses, allows for easier time predicting, stall out for residual damage(eg. bad poison is useful in conjunction with Mega Gengar's trapping ability), and allows you to sustain a hit from the next incoming threat after you kill off a pokemon, given that you are unable to 1HKO the threat in 1 turn.